Ground substrate and method for producing same

ABSTRACT

Provided is a base substrate including an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element. A front surface of the orientation layer on a side used for crystal growth is composed of a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire. The orientation layer contains a solid solution containing two or more selected from the group consisting of α-Al2O3, α-Cr2O3, α-Fe2O3, α-Ti2O3, α-V2O3, and α-Rh2O3.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of PCT/JP2019/038594 filed Sep. 30, 2019, which claims priority to Japanese Patent Application No. 2019-063601 filed Mar. 28, 2019, the entire contents all of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a base substrate used for crystal growth of a nitride or oxide of a Group 13 element and a method for producing the same.

2. Description of the Related Art

In recent years, semiconductor devices using gallium nitride (GaN) have been put into practical use. For example, a device in which an n-type GaN layer, a multiple quantum well layer (MQW) in which a quantum well layer composed of an InGaN layer and a barrier layer composed of a GaN layer are alternately stacked, and a p-type GaN layer are sequentially stacked on a sapphire substrate has been mass-produced.

Further, research and development of corundum phase type a-gallium oxide (α-Ga₂O₃), which has the same crystal structure as sapphire, is being actively carried out. In fact, α-Ga₂O₃ has a large bandgap of 5.3 eV, and is expected as a material for power semiconductor elements. For example, Patent Literature 1 (JP2014-72533A) discloses an example in which α-Ga₂O₃ is formed as a semiconductor layer on a sapphire substrate in a semiconductor device formed of a base substrate having a corundum-type crystal structure, a semiconductor layer having a corundum-type crystal structure, and an insulating film having a corundum-type crystal structure. Further, Patent Literature 2 (JP2016-25256A) discloses a semiconductor device including an n-type semiconductor layer containing a crystalline oxide semiconductor having a corundum structure as a main component, a p-type semiconductor layer containing an inorganic compound having a hexagonal crystal structure as a main component, and an electrode, in which a diode is fabricated by forming α-Ga₂O₃ having a corundum structure which is a metastable phase as an n-type semiconductor layer and an α-Rh₂O₃ film having a hexagonal crystal structure as a p-type semiconductor layer on a c-plane sapphire substrate in an example.

It is known that, in these semiconductor devices, better characteristics can be obtained when there are fewer crystal defects in the material. In particular, since a power semiconductor is required to have excellent withstand voltage characteristics, it is desirable to reduce crystal defects. This is because the dielectric breakdown electric field characteristics depend on the number of crystal defects. However, in the case of GaN and α-Ga₂O₃, a single-crystal substrate having few crystal defects has not been put into practical use, and the single crystal substrate is generally formed by heteroepitaxial growth on a sapphire substrate having different lattice constants from those of these materials.

Therefore, crystal defects due to the difference in lattice constant with sapphire are likely to occur. For example, in a case where α-Ga₂O₃ is formed on the c-plane of sapphire, the a-axis length (4.754 Å) of sapphire (α-Al₂O₃) and the a-axis length (4.983 Å) of α-Ga₂O₃ differ by about 4.8%, and this difference is the main cause of crystal defects.

It has been reported that defects are reduced by forming a buffer layer between the sapphire and the α-Ga₂O₃ layer when forming the α-Ga₂O₃ layer as an approach for reducing crystal defects by reducing the difference in lattice constants between the sapphire and the semiconductor layer. For example, Non-Patent Literature 1 (Applied Physics Express, vol. 9, pages 071101-1 to 071101-4) discloses an example in which edge dislocations and screw dislocations are 3×10⁸/cm² and 6×10⁸/cm², respectively, by introducing a (Al_(x), Ga_(1-x))₂O₃ layer (x=0.2 to 0.9) as a buffer layer between sapphire and an α-Ga₂O₃ layer. Further, Non-Patent Literature 2 (Applied Physics Express 11, 111101 (2018)) discloses a substrate in which an α-Cr₂O₃ film was formed as a buffer layer on sapphire as a substrate for forming an α-Ga₂O₃ film.

CITATION LIST PATENT LITERATURE

Patent Literature 1: JP2014-72533A

Patent Literature 2: JP2016-25256A

NON-PATENT LITERATURE

Non-Patent Literature 1: Riena Jinno et al., Reduction in edge dislocation density in corundum-structured α-Ga₂O₃ layers on sapphire substrates with quasi-graded α-(AL,Ga)₂O₃ buffer layers, Applied Physics Express, Japan, The Japan Society of Applied Physics, Jun. 1, 2016, vol. 9, pages 071101-1 to 071101-4

Non-Patent Literature 2: Giang T. Dang et al., Growth of α-Cr₂O₃ single crystals by mist CVD using ammonium dichromate, Applied Physics Express 11, 111101 (2018)

SUMMARY OF THE INVENTION

However, the methods of introducing a buffer layer as disclosed in Non-Patent Literature 1 and Non-Patent Literature 2 are insufficient for application to a power semiconductor requiring high dielectric breakdown electric field characteristics, and it is desirable to further reduce crystal defects. Even in a case where α-Cr₂O₃ as disclosed in Non-Patent Literature 2 is formed as a buffer layer, it is insufficient for application to a power semiconductor requiring high dielectric breakdown electric field characteristics, and it is desirable to further reduce crystal defects. In a case where an a-Cr₂O3 film is used as the buffer layer, it is presumed that the reason why crystal defects cannot be sufficiently reduced is that i) there is a lattice mismatch between α-Cr₂O₃ and α-Ga₂O₃, and ii) a thin buffer layer is formed on sapphire by heteroepitaxial growth, and therefore large crystal defects are contained in the buffer layer.

The present inventors have now found that an excellent semiconductor layer can be formed by using a base substrate including an orientation layer in which a front surface on a side used for crystal growth of a nitride or oxide of a Group 13 element is composed of a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire, and the orientation layer contains a solid solution containing two or more selected from the group consisting of α-Al₂O₃, α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃.

Therefore, an object of the present invention is to provide a high-quality base substrate including an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element, in which crystal defects (dislocations) in the orientation layer are significantly reduced. Further, another object of the present invention is to provide a method for producing such a base substrate.

According to an aspect of the present invention, there is provided a base substrate comprising an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element,

-   -   wherein a front surface of the orientation layer on a side used         for the crystal growth is composed of a material having a         corundum-type crystal structure having an a-axis length and/or         c-axis length larger than that of sapphire, and     -   wherein the orientation layer contains a solid solution         comprising two or more selected from the group consisting of         α-Al₂O₃, α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃.

According to another aspect of the present invention, there is provided a method for producing the base substrate, comprising:

-   -   providing a sapphire substrate;     -   forming an orientation precursor layer on a surface of the         sapphire substrate, wherein the orientation precursor layer         contains a material having a corundum-type crystal structure         having an a-axis length and/or c-axis length larger than that of         sapphire, or a material capable of having a corundum-type         crystal structure having an a-axis length and/or c-axis length         larger than that of sapphire by heat treatment; and     -   heat-treating the sapphire substrate and the orientation         precursor layer at a temperature of 1000° C. or greater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a configuration of an aerosol deposition (AD) apparatus.

FIG. 2 is a schematic cross-sectional view showing a configuration of a mist CVD apparatus.

FIG. 3 is a diagram schematically showing a step of preparing a composite base substrate in Example 1.

FIG. 4 is a diagram schematically showing a step of preparing a sample for evaluating a rear surface of an orientation layer in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

Base Substrate

A base substrate according to the present invention is a base substrate comprising an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element. That is, this base substrate is used for crystal growth of a semiconductor layer composed of a nitrides or oxide of a Group 13 element on the orientation layer. Here, the term “Group 13 element” refers to a Group 13 element according to the periodic table formulated by the IUPAC (International Union of Pure and Applied Chemistry), and specifically, is any one of boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (TI), and nihonium (Nh). The nitride and oxide of Group 13 element are typically gallium nitride (GaN) and α-gallium oxide (α-Ga₂O₃).

The orientation layer has a structure in which the crystal orientations are substantially aligned in the substantially normal direction. With such a configuration, it is possible to form a semiconductor layer having excellent quality, particularly excellent orientation, on the orientation layer. That is, when the semiconductor layer is formed on the orientation layer, the crystal orientation of the semiconductor layer generally follows the crystal orientation of the orientation layer. Therefore, when the base substrate includes an orientation layer, the semiconductor film can serve as an orientation film. The orientation layer may be a polycrystal, a mosaic crystal (a set of crystals of which crystal orientations are slightly deviated), or a single-crystal. In a case where the orientation layer is polycrystalline, it is preferably a biaxial orientation layer in which the twist direction (that is, the rotation direction about the substrate normal oriented substantially perpendicular to the substrate surface) is also substantially aligned.

The front surface of the orientation layer on the side used for crystal growth (hereinafter, may be simply referred to as “front surface” or “orientation layer front surface”) is composed of a material having a corundum-type crystal structure having an a-axis length and/or a c-axis length larger than that of sapphire (α-Al₂O₃). By controlling the lattice constant of the orientation layer in this way, it is possible to significantly reduce the crystal defects in the semiconductor layer formed on the orientation layer. That is, the oxide of the Group 13 elements constituting the semiconductor layer have a lattice constant larger than that of sapphire (α-Al₂O₃). In fact, as shown in Table 1 below, the lattice constant (a-axis length and c-axis length) of α-Ga₂O₃, which is an oxide of Group 13 elements, are larger than the lattice constant of α-Al₂O₃. Therefore, by controlling the lattice constant of the orientation layer to be larger than that of α-Al₂O₃, when the semiconductor layer is formed on the orientation layer, the mismatch of the lattice constant between the semiconductor layer and the orientation layer is decreased, and as a result, crystal defects in the semiconductor layer are reduced. For example, in a case where α-Ga₂O₃ is formed on the c-plane of sapphire, the lattice length (a-axis length) in the in-plane direction of α-Ga₂O₃ is larger than that of sapphire, and there is a mismatch of about 4.8%. Therefore, by controlling the a-axis length of the orientation layer to be larger than that of α-Al₂O₃, crystal defects in the α-Ga₂O₃ layer can be reduced. In a case where α-Ga₂O₃ is formed on the m-plane of sapphire, the lattice length (c-axis length and a-axis length) in the in-plane direction of α-Ga₂O₃ is larger than that of sapphire, and there is a mismatch of about 3.4% in the c-axis length and about 4.8% in the a-axis length. Therefore, by controlling the c-axis length and the a-axis length of the orientation layer to be larger than that of α-Al₂O₃, crystal defects in the α-Ga₂O₃ layer can be reduced. Further, GaN, which is a nitride of Group 13 elements, also has a large mismatch between sapphire and lattice constant. In a case where GaN is formed on the c-plane of sapphire, the lattice length (a-axis length) in the in-plane direction of GaN is substantially larger than that of sapphire, and there is a mismatch of about 16.2%. Therefore, by controlling the a-axis length of the orientation layer to be larger than that of α-Al₂O₃, crystal defects in the GaN layer can be reduced. On the other hand, when these semiconductor layers are formed directly on the sapphire substrate, stress is generated in the semiconductor layer due to the mismatch of lattice constants, and a large amount of crystal defects may be generated in the semiconductor layer.

TABLE 1 Lattice constants of Group 13 oxide a-axis length c-axis length (A) (A) α-Ga₂O₃ 4.983 13.433 α-Al₂O₃ 4.754 12.990 α-Cr₂O₃ 4.961 13.599 α-Fe₂O₃ 5.038 13.772 α-Ti₂O₃ 5.157 13.610 α-V₂O₃ 4.949 13.998 α-Rh₂O₃ 5.127 13.853 α-ln₂O₃ 5.487 14.510

It is preferable that the entirety of the orientation layer is composed of a material having a corundum-type crystal structure. By doing so, it is possible to reduce crystal defects in the orientation layer and the semiconductor layer. It is desirable that the orientation layer is formed on the surface of the sapphire substrate. α-Al₂O₃ constituting the sapphire substrate has a corundum-type crystal structure, and since the orientation layer is composed of a material having a corundum-type crystal structure, the crystal structure can be made the same as that of the sapphire substrate, and as a result, the occurrence of crystal defects in the orientation layer due to crystal structure mismatch is suppressed. In this respect, it is preferable that the crystal defects in the orientation layer are reduced because the crystal defects in the semiconductor layer formed on the orientation layer are also reduced. This is because when a large number of crystal defects are present in the orientation layer, the crystal defects are also taken over to the semiconductor layer formed thereon, and as a result, crystal defects also occur in the semiconductor layer.

The material having a corundum-type crystal structure constituting the orientation layer preferably contains a solid solution containing two or more selected from the group consisting of α-Al₂O₃, α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃. As shown in Table 1, these materials have lattice constants (a-axis length and/or c-axis length) larger than that of α-Al₂O₃, and the lattice constants are relatively close to or coincide with those of GaN and α-Ga₂O₃, which are a nitride and an oxide of Group 13 elements constituting the semiconductor layer, so that crystal defects in the semiconductor layer can be effectively suppressed. The solid solution may be a substitutional solid solution or an interstitial solid solution, but is preferably a substitutional solid solution. Incidentally, the orientation layer is composed of a material having a corundum-type crystal structure, but this does not preclude the inclusion of other trace components.

A particularly preferable orientation layer is composed of a material containing a solid solution of α-Cr₂O₃ and a different material. In this case, the base substrate (particularly, orientation layer) can be preferably used for crystal growth of semiconductor layers composed of α-Ga₂O₃ or an α-Ga₂O₃ solid solution. That is, in the crystal growth of α-Ga₂O₃ or an α-Ga₂O₃ solid solution, by using a base substrate having an orientation layer in which the front surface on a side used for film formation is composed of at least a solid solution of α-Cr₂O₃ and a different material, an excellent semiconductor layer composed of α-Ga₂O₃ or an α-Ga₂O₃ solid solution can be formed thereon.

The a-axis length of the material having a corundum-type crystal structure on the front surface of the orientation layer on the side used for crystal growth is larger than 4.754 Å and 5.157 Å or less, and more preferably 4.850 to 5.000 Å. It is still more preferably 4.900 to 5.000 Å. The c-axis length of the material having a corundum-type crystal structure on the front surface of the orientation layer on the side used for crystal growth is larger than 12.990 Å and 13.998 Å or less, and more preferably 13.000 to 13.800 Å. It is still more preferably 13.400 to 13.600 Å. By controlling within such a range, the a-axis length and/or the c-axis length of the front surface of the orientation layer can be close to the lattice constant (a-axis length and/or c-axis length) of the nitride or oxide of Group 13 element constituting the semiconductor layer, in particular, α-Ga₂O₃.

The thickness of the orientation layer is preferably 10 μm or more, and more preferably 40 μm or more. The upper limit of the thickness is not particularly limited, but is typically 1000 μm or less. In a case where the orientation layer is used as a stand-alone substrate, the orientation layer may be even thicker from the viewpoint of handleability, and the thickness may be, for example, 1 mm or more, and from the viewpoint of cost, the thickness may be, for example, 2 mm or less. The crystal defects on the front surface of the orientation layer can also be reduced by increasing the thickness of the orientation layer. When the orientation layer is formed on the sapphire substrate, the lattice constant of the sapphire substrate is slightly different from that of the orientation layer, and as a result, crystal defects are likely to occur at the interface between the sapphire substrate and the orientation layer, that is, in the lower portion of the orientation layer. However, by increasing the thickness of the orientation layer, it is possible to reduce the influence of such crystal defects generated in the lower portion of the orientation layer on the front surface of the orientation layer. The reason for this is not clear, but it is considered that the crystal defects generated in the lower portion of the orientation layer do not reach the front surface of the thick orientation layer and disappear. In addition, by increasing the thickness of the orientation layer, it is expected that the semiconductor layer can be separated after the semiconductor layer is formed over the orientation layer, and the base substrate can be reused. The crystal defect density on the front surface of the orientation layer is preferably 1.0×10⁸/cm² or less, more preferably 1.0×10⁶/cm² or less, still more preferably 4.0×10³/cm² or less, and there is no particular lower limit. In the present specification, the crystal defects refer to threading edge dislocations, threading screw dislocations, threading mixed dislocations, and basal plane dislocations, and the crystal defect density is the total of the dislocation densities. The basal plane dislocation is a problem in a case where the base substrate including the orientation layer has an off-angle, and is not a problem because the front surface of the orientation layer is not exposed in a case where there is no off-angle. For example, when the threading edge dislocations are 3×10⁸/cm², the threading screw dislocations are 6×10⁸/cm², and the threading mixed dislocations are 4×10⁸/cm² in the material, the crystal defect density becomes 1.3×10⁹/cm².

In a case where the orientation layer is formed on a sapphire substrate, a gradient composition region having a composition varying in a thickness direction is preferably present in the orientation layer. For example, the gradient composition region preferably includes a region which is composed of a solid solution of one or more materials selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃, and α-Al₂O₃, and has a gradient composition in which the solid solution amount of α-Al₂O₃ decreases from the sapphire substrate side toward the orientation layer front surface side. The gradient composition region is preferably composed of a solid solution containing α-Al₂O₃ and α-Cr₂O₃, and particularly preferably composed of a solid solution containing α-Al₂O₃, α-Cr₂O₃, and α-Ti₂O₃, or a solid solution containing α-Al₂O₃, α-Cr₂O₃, α-Ti₂O₃, and α-Fe₂O₃. That is, it is desirable that the orientation layer is formed on the surface of the sapphire substrate, but stress due to a difference in lattice constant (a-axis length and/or c-axis length) between the sapphire substrate and the orientation layer is relaxed, and there is an effect of suppressing generation of crystal defects. In other words, it is preferable that the a-axis length and/or c-axis length is different between the front surface and the rear surface of the orientation layer, and it is preferable that the front surface has a larger a-axis length and/or c-axis length than the a-axis length and/or c-axis length on the rear surface of the orientation layer. With such a structure, the lattice constant varies in the thickness direction even though the orientation layer is a single-crystal, a mosaic crystal, or a biaxial orientation layer. The difference between the a-axis length and/or the c-axis length of the front surface and the rear surface of the orientation layer is preferably 2.5% or more, more preferably 4.0% or more (the upper limit of this value is not particularly limited, but is typically 15.4% or less). Therefore, a single-crystal, a mosaic crystal, or a biaxially orientation layer can be formed on substrates having different lattice constants in a state where the stress is relaxed. Such a gradient composition region can be formed by heat-treating the sapphire substrate and the orientation precursor layer at a temperature of 1000° C. or greater in the production of the base substrate described later. That is, when heat treatment is performed at such a high temperature, a reaction occurs at the interface between the sapphire substrate and the orientation precursor layer, and the Al component in the sapphire substrate diffuses into the orientation precursor layer, and the component in the orientation precursor layer diffuses into the sapphire substrate. As a result, a gradient composition region is formed in which the solid solution amount of α-Al₂O₃ varies in the thickness direction. The thicker the gradient composition region, the easier it is to relax the stress caused by the difference in lattice constants, so the thicker the composition region, the better. Therefore, the thickness of the gradient composition region is preferably 5 μm or more, more preferably 20 μm or more. The upper limit of the thickness is not particularly limited, but is typically 100 μm or less. Further, by performing the heat treatment at 1000° C. or greater, the crystal defects reaching the front surface of the orientation layer can be effectively reduced. The reason for this is not clear, but it is considered that heat treatment at high temperature promotes annihilation of crystal defects.

According to a more preferred embodiment of the present invention, the orientation layer has a composition stable region located near the front surface and having a composition stable in a thickness direction, and a gradient composition region located far from the front surface and having a composition varying in the thickness direction. The composition stable region is a region in which the change in the content ratio of each metal element is less than 1.0 at%, and the gradient composition region is a region in which the change in the content ratio of each metal element is 1.0 at % or more. For example, the composition stable region and the gradient composition region can be determined as follows. First, a cross-sectional sample of the orientation layer is provided, and an energy dispersive X-ray analysis (EDS) is performed at any ten locations in the vicinity of the front surface of the orientation layer, and an average value of the content ratio (at %) of the detected metal element is calculated. Next, EDS analysis is performed at arbitrary ten points apart from the front surface by 2 μm in the thickness direction, and the average value of the content ratio (at %) at the 2 μm thickness point is calculated. At this time, the average value of the content ratio between the front surface and the point of 2 μm thickness is compared, and the difference in the content ratio of at least one kind of metal element among all detected metal elements is less than 1.0 at% or 1.0 at % or more, and can be assigned to either the composition stable region or the gradient composition region for the region from the front surface to 2 μm thickness. In the same way, by calculating the average value of the content ratio of the metal element for every 2 μm in the thickness direction and comparing the average value of the content ratio of the metal element between a certain thickness point and a point separated by 2 μm in the thickness direction, it is possible to determine the attribution of the region between the points. For example, the region between the point having a thickness of 24 μm and the point having a thickness of 26 μm from the front surface can be assigned by calculating the average value of the metal element content ratio at each point and comparing them. For example, in a case where the orientation layer contains Al, it is more preferable that the Al concentration in the gradient composition region decreases in the thickness direction toward the composition stable region. In this embodiment, the material having a corundum-type crystal structure is preferably a solid solution containing two or more selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, and α-Ti₂O₃, or a solid solution containing α-Al₂O₃ and one or more materials selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, and α-Ti₂O₃. Particularly preferably, the gradient composition region is composed of a solid solution containing α-Cr₂O₃ and α-Al₂O₃. The composition stable region may be a material having a lattice constant (a-axis length and/or c-axis length) larger than that of α-Al₂O₃, may be a solid solution containing a plurality of corundum-type materials, or may be a corundum-type material single phase. That is, the material constituting the composition stable region is preferably (i) a solid solution containing two or more materials selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃, or (ii) a solid solution of one or more selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃, and α-Al₂O₃. From the viewpoint of controlling the lattice constant, it is preferable that the material is composed of a solid solution of a material that does not contain α-Al₂O₃ among the above materials.

The material constituting the orientation layer is not particularly limited as long as it has an orientation property with respect to the surface of the base substrate, and is, for example, c-axis oriented, a-axis oriented, or m-axis oriented. By doing so, when the semiconductor layer is formed on the base substrate, the semiconductor film can be oriented in the c-axis orientation film, the a-axis orientation film, or the m-axis orientation film.

The orientation layer is preferably a heteroepitaxial growth layer. For example, in a case where the orientation layer is grown on the sapphire substrate, since both the sapphire substrate and the orientation layer have a corundum-type crystal structure, epitaxial growth in which the crystal plane of the orientation layer is arranged according to the crystal orientation of the sapphire substrate may occur during the heat treatment in a case where the lattice constants of the sapphire substrate and the orientation layer are close to each other. By epitaxially growing the orientation layer in this way, the orientation layer can inherit the high crystallinity and crystal orientation peculiar to the single-crystal of the sapphire substrate.

The arithmetical mean roughness Ra on the front surface of the orientation layer is preferably 1 nm or less, more preferably 0.5 nm or less, and still more preferably 0.2 nm or less. It is considered that the crystallinity of the semiconductor layer provided thereon is further improved by smoothing the front surface of the orientation layer in this way.

The base substrate has an area of preferably 20 cm² or more, more preferably 70 cm² or more, and still more preferably 170 cm² or more on one side thereof. By increasing the area of the base substrate in this way, it is possible to increase the area of the semiconductor layer formed on the substrate. Therefore, it is possible to obtain a large number of semiconductor elements from one semiconductor layer, and further reduction in production cost is expected. The upper limit of the size is not particularly limited, but is typically 700 cm² or less on one side.

The base substrate of the present invention preferably further includes a support substrate on the side opposite to the front surface (that is, the rear surface side) of the orientation layer. That is, the base substrate of the present invention may be a base substrate including a support substrate and an orientation layer provided on the support substrate. The support substrate is preferably a sapphire substrate or a corundum single-crystal such as Cr₂O3, and particularly preferably a sapphire substrate. By using a corundum single-crystal as the support substrate, the orientation layer can also serve as a seed crystal for heteroepitaxial growth. Further, by forming the structure including the corundum single-crystal as described above, a semiconductor layer having excellent quality can be obtained. That is, the corundum single-crystal has characteristics such as excellent mechanical properties, thermal properties, and chemical stability. In particular, sapphire has a high thermal conductivity of 42 W/m·K at room temperature and is excellent in thermal conductivity. Therefore, by using a base substrate including a sapphire substrate, the thermal conductivity of the entire substrate can be improved. As a result, when the semiconductor layer is formed on the base substrate, it is expected that the temperature distribution in the substrate surface is prevented from becoming non-uniform, and the semiconductor layer having a uniform film thickness can be obtained. Further, a sapphire substrate having a large area is easily available, so that the overall cost can be reduced and a semiconductor layer having a large area can be obtained.

The sapphire substrate used as the support substrate may have any orientation plane. That is, the sapphire substrate may have an a-plane, a c-plane, an r-plane, or an m-plane, or may have a predetermined off-angle with respect to these planes. Further, sapphire to which a dopant is added may be used in order to adjust the electrical characteristics. As such a dopant, a known dopant can be used.

A semiconductor layer composed of a nitride or oxide of Group 13 element can be formed using the orientation layer of the base substrate according to the present invention. The semiconductor layer can be formed by a known method, but is preferably formed by any of vapor phase film forming methods such as various CVD methods, an HVPE method, a sublimation method, an MBE method, a PLD method, and a sputtering method, and liquid phase film forming methods such as a hydrothermal method and an Na flux method. Examples of the CVD method include a thermal CVD method, a plasma CVD method, a mist CVD method, and an MO (metal organic) CVD method. Among these, the mist CVD method, the hydrothermal method, or the HVPE method is particularly preferable for forming the semiconductor layer composed of the oxide of Group 13 element.

The base substrate of the present invention may be in the form of a self-standing substrate having an orientation layer alone, or may be in the form of a base substrate with a support substrate such as a sapphire substrate. Therefore, if desired, the orientation layer may ultimately be separated from the support substrate, such as a sapphire substrate. The separation of the support substrate may be performed by a known method and is not particularly limited. Examples thereof include a method of separating an orientation layer by applying mechanical impact, a method of separating an orientation layer by applying heat and utilizing thermal stress, a method of separating an orientation layer by applying vibration such as ultrasonic waves, a method of separating an orientation layer by etching an unnecessary portion, a method of separating an orientation layer by laser lift-off, and a method of separating an orientation layer by mechanical processing such as cutting or polishing. Further, in the case of heteroepitaxially growing the orientation layer on the sapphire substrate, the orientation layer may be installed on another support substrate after the sapphire substrate is separated. The material of the other support substrate is not particularly limited, but a suitable material may be selected from the viewpoint of material properties. For example, from the viewpoint of thermal conductivity, a metal substrate or a substrate made of Cu or the like, a ceramic substrate made of SiC, AlN or the like, or the like may be used.

Production Method

The base substrate of the present invention can be preferably produced by (a) providing a sapphire substrate, (b) preparing a predetermined orientation precursor layer, (c) performing heat treatment on the orientation precursor layer on the sapphire substrate to convert at least a portion near the sapphire substrate into an orientation layer, and optionally (d) subjecting the orientation layer to processing such as grinding or polishing to expose the surface of the orientation layer. This orientation precursor layer becomes an orientation layer by heat treatment and contains a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire, or a material capable of having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire by heat treatment to be described later. Further, the orientation precursor layer may contain trace components in addition to the material having a corundum-type crystal structure. According to such a production method, the growth of the orientation layer can be promoted by using the sapphire substrate as a seed crystal. That is, the high crystallinity and crystal orientation peculiar to the single-crystal of the sapphire substrate are inherited by the orientation layer.

(a) Provision of Sapphire Substrate

To prepare a base substrate, first, a sapphire substrate is provided. The sapphire substrate used may have any orientation plane. That is, the sapphire substrate may have an a-plane, a c-plane, an r-plane, or an m-plane, or may have a predetermined off-angle with respect to these planes. For example, in a case where a c-plane sapphire is used, since the c-axis is oriented with respect to the surface, it is possible to easily heteroepitaxially grow a c-axis oriented orientation layer thereon. A sapphire substrate to which a dopant is added may also be used to adjust electrical properties. As such a dopant, a known dopant can be used.

(b) Preparation of Orientation Precursor Layer

An orientation precursor layer containing a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire, or a material capable of having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire by heat treatment is prepared. The method for forming the orientation precursor layer is not particularly limited, and a known method can be adopted. Examples of the method for forming the orientation precursor layer include an aerosol deposition (AD) method, a sol-gel method, a hydrothermal method, a sputtering method, an evaporation method, various chemical vapor deposition (CVD) methods, an HVPE method, a PLD method, a chemical vapor transport (CVT) method, and a sublimation method. Examples of the CVD method include a thermal CVD method, a plasma CVD method, a mist CVD method, and an MO (metal organic) CVD method. Alternatively, a method may be used in which a molded body of the orientation precursor is prepared in advance and the molded body is placed on a sapphire substrate. Such a molded body can be produced by molding the material of the orientation precursor by a method such as tape casting or press molding. Further, it is also possible to use a method in which a polycrystal prepared in advance by various CVD methods, sintering, or the like is used as the orientation precursor layer and is placed on a sapphire substrate.

However, aerosol deposition (AD) methods, various CVD methods, or sputtering methods are preferred. By using these methods, a dense orientation precursor layer can be formed in a relatively short time, and heteroepitaxial growth using a sapphire substrate as a seed crystal can be easily caused. In particular, the AD method does not require a high vacuum process and has a relatively high film formation rate, and is therefore preferable in terms of production cost. In the case of using a sputtering method, a film can be formed using a target of the same material as that of the orientation precursor layer, but a reactive sputtering method in which a film is formed in an oxygen atmosphere using a metal target can also be used. A method of placing a molded body prepared in advance on sapphire is also preferable as a simple method, but since the orientation precursor layer is not dense, a process of densification is required in the heat treatment step described later. In the method of using a polycrystalline prepared in advance as an orientation precursor layer, two steps of a step of preparing a polycrystalline body and a step of performing heat treatment on a sapphire substrate are required. Further, in order to improve the adhesion between the polycrystal and the sapphire substrate, it is necessary to take measures such as keeping the surface of the polycrystal sufficiently smooth. Although known conditions can be used for any of the methods, a method of directly forming an orientation precursor layer using an AD method and a method of placing a molded body prepared in advance on a sapphire substrate will be described below.

The AD method is a technique for forming a film by mixing fine particles or a fine particle raw material with a gas to form an aerosol, and impacting the aerosol on a substrate by injecting the aerosol at a high speed from a nozzle, and has a feature of forming a film densified at ordinary temperature. FIG. 1 shows an example of a film forming apparatus (aerosol deposition (AD) apparatus) used in such an AD method. The film forming apparatus 20 shown in FIG. 1 is configured as an apparatus used in an AD method in which a raw material powder is injected onto a substrate in an atmosphere having a pressure lower than atmospheric pressure. The film forming apparatus 20 includes an aerosol generating unit 22 that generates an aerosol of raw material powder containing raw material components, and a film forming unit 30 that forms a film containing the raw material components by injecting the raw material powder onto the sapphire substrate 21. The aerosol generating unit 22 includes an aerosol generating chamber 23 that stores raw material powder and receives a carrier gas supply from a gas cylinder (not shown) to generate an aerosol, and a raw material supply pipe 24 that supplies the generated aerosol to the film forming unit 30, and a vibrator 25 that applies vibration at frequencies of 10 to 100 Hz to the aerosol generating chamber 23 and the aerosol therein. The film forming unit 30 has a film forming chamber 32 that injects aerosols onto the sapphire substrate 21, a substrate holder 34 that is disposed inside the film forming chamber 32 and fixes the sapphire substrate 21, and an X-Y stage 33 that moves the substrate holder 34 in the X-Y axis direction. Further, the film forming unit 30 includes an injection nozzle 36 in which a slit 37 is formed at a tip thereof to inject aerosol into the sapphire substrate 21, and a vacuum pump 38 for reducing the pressure in the film forming chamber 32.

It is known that the AD method can control a film thickness, a film quality, and the like according to film forming conditions. For example, the form of the AD film is easily affected by the collision rate of the raw material powder to the substrate, the particle size of the raw material powder, the aggregated state of the raw material powder in the aerosol, the injection amount per unit time, and the like. The collision rate of the raw material powder with the substrate is affected by the differential pressure between the film forming chamber 32 and the injection nozzle 36, the opening area of the injection nozzle, and the like. If appropriate conditions are not used, the coating may become a green compact or generate pores, so it is necessary to appropriately control these factors.

In a case where a molded body in which the orientation precursor layer is prepared in advance is used, the raw material powder of the orientation precursor can be molded to prepare the molded body. For example, in a case where press molding is used, the orientation precursor layer is a press molded body. The press molded body can be prepared by press-molding the raw material powder of the orientation precursor based on a known method, and may be prepared, for example, by placing the raw material powder in a mold and pressing the raw material powder at pressures of preferably 100 to 400 kgf/cm², and more preferably 150 to 300 kgf/cm². The molding method is not particularly limited, and in addition to press molding, tape casting, slip casting, extrusion molding, doctor blade method, and any combination thereof can be used. For example, in the case of using tape casting, it is preferable that additives such as a binder, a plasticizer, a dispersant, and a dispersion medium are appropriately added to the raw material powder to form a slurry, and the slurry is discharged and molded into a sheet shape by passing through a slit-shaped thin discharge port. The thickness of the molded body formed into a sheet is not limited, but is preferably 5 to 500 μm from the viewpoint of handling. Further, in a case where a thick orientation precursor layer is required, a large number of these sheet molded bodies may be stacked and used as a desired thickness.

In these molded bodies, the portion near the sapphire substrate becomes an orientation layer by the subsequent heat treatment on the sapphire substrate. As described above, in such a method, it is necessary to sinter and densify the molded body in the heat treatment step described later. Therefore, the molded body may contain trace components such as a sintering aid in addition to the material having or resulting in a corundum-type crystal structure.

(c) Heat Treatment of Orientation Precursor Layer on Sapphire Substrate

A heat treatment is performed on the sapphire substrate on which the orientation precursor layer is formed at a temperature of 1000° C. or greater. By this heat treatment, at least a portion of the orientation precursor layer near the sapphire substrate can be converted into a dense orientation layer. Further, this heat treatment enables heteroepitaxial growth of the orientation layer. That is, by forming the orientation layer with a material having a corundum-type crystal structure, heteroepitaxial growth occurs in which the material having a corundum-type crystal structure crystal grows using a sapphire substrate as a seed crystal during heat treatment. At that time, the crystals are rearranged, and the crystals are arranged according to the crystal plane of the sapphire substrate. As a result, the crystal axes of the sapphire substrate and the orientation layer can be aligned. For example, when a c-plane sapphire substrate is used, both the sapphire substrate and the orientation layer can be c-axis oriented with respect to the surface of the base substrate. Moreover, this heat treatment makes it possible to form a gradient composition region in a part of the orientation layer. That is, during the heat treatment, a reaction occurs at the interface between the sapphire substrate and the orientation precursor layer, and the Al component in the sapphire substrate diffuses into the orientation precursor layer, and/or the component in the orientation precursor layer diffuses into the sapphire substrate, thereby forming a gradient composition region composed of a solid solution containing α-Al₂O₃.

It is known that methods such as various CVD methods, a sputtering method, an HVPE method, a PLD method, a CVT method, and a sublimation method may cause heteroepitaxial growth on a sapphire substrate without heat treatment at 1000° C. or greater. However, it is preferable that the orientation precursor layer is in a non-oriented state, that is, amorphous or non-oriented polycrystalline, at the time of preparation thereof, and the crystal rearrangement is caused by using sapphire as a seed crystal at the time of the heat treatment step. By doing so, it is possible to effectively reduce the crystal defects that reach the front surface of the orientation layer. The reason for this is not clear, but it is considered that the crystal defects generated in the lower portion of the orientation layer are likely to be annihilated.

The heat treatment is not particularly limited as long as a corundum-type crystal structure is obtained and heteroepitaxial growth using a sapphire substrate as a seed occurs, and can be performed in a known heat treatment furnace such as a tubular furnace or a hot plate. Further, in addition to the heat treatment under normal pressure (without pressing), a heat treatment under pressure such as hot pressing or HIP, or a combination of a heat treatment under normal pressure and a heat treatment under pressure can also be used. The heat treatment conditions can be appropriately selected depending on the material used for the orientation layer. For example, the atmosphere of the heat treatment can be selected from the air, vacuum, nitrogen and inert gas atmosphere. The preferred heat treatment temperature also varies depending on the material used for the orientation layer, but is preferably 1000 to 2000° C., and more preferably 1200 to 2000° C., for example. The heat treatment temperature and the retention time are related to the thickness of the orientation layer formed by heteroepitaxial growth and the thickness of the gradient composition region formed by diffusion with the sapphire substrate, and can be appropriately adjusted depending on the kind of the material, the target orientation layer, the thickness of the gradient composition region, and the like. However, in the case of using molded body prepared in advance is used as the orientation precursor layer, it is necessary to perform sintering and densification during heat treatment, and normal pressure firing at a high temperature, hot pressing, HIP, or a combination thereof is suitable. For example, when using a hot press, the surface pressure is preferably 50 kgf/cm² or more, more preferably 100 kgf/cm² or more, particularly preferably 200 kgf/cm² or more, the upper limit is not particularly limited. The firing temperature is also not particularly limited as long as sintering, densification, and heteroepitaxial growth occur, but is preferably 1000° C. or greater, more preferably 1200° C. or greater, still more preferably 1400° C. or greater, and particularly preferably 1600° C. or greater. The firing atmosphere can also be selected from atmosphere, vacuum, nitrogen and an inert gas atmosphere. As the firing jig such as a mold, those made of graphite or alumina can be used.

(d) Exposure of Surface of Orientation Layer

On the orientation layer formed near the sapphire substrate by the heat treatment, an orientation precursor layer or a surface layer having poor orientation or no orientation may exist or remain. In this case, it is preferable that the surface derived from the orientation precursor layer is subjected to processing such as grinding or polishing to expose the surface of the orientation layer. By doing so, a material having excellent orientation is exposed on the surface of the orientation layer, so that the semiconductor layer can be effectively epitaxially grown on the material. The method for removing the orientation precursor layer and the surface layer is not particularly limited, and examples thereof include a method for grinding and polishing and a method for ion beam milling. The surface of the orientation layer is preferably polished by lapping using abrasive grains or chemical mechanical polishing (CMP).

Semiconductor Layer

A semiconductor layer composed of a nitride or oxide of Group 13 element can be formed using the base substrate of the present invention. The semiconductor layer can be formed by a known method, but is preferably formed by any of vapor phase film forming methods such as various CVD methods, an HVPE method, a sublimation method, an MBE method, a PLD method, and a sputtering method, and liquid phase film forming methods such as a hydrothermal method and an Na flux method, and a mist CVD method, a hydrothermal method, or a HVPE method is particularly preferable. The mist CVD method will be described below.

The mist CVD method is a method in which a raw material solution is atomized or formed into droplets to generate mist or droplets, the mist or droplets are transported to a film forming chamber including a substrate using a carrier gas, and the mist or droplets are thermally decomposed and chemically reacted in the film forming chamber to form and grow a film on the substrate, and thus does not require a vacuum process and can prepare a large number of samples in a short time. Here, FIG. 2 shows an example of a mist CVD apparatus. The mist CVD apparatus 61 shown in FIG. 2 includes a susceptor 70 on which a substrate 69 is placed, a dilution gas source 62 a, a carrier gas source 62 b, a flow control valve 63 a for controlling a flow rate of a dilution gas sent from the dilution gas source 62 a, a flow control valve 63 b for controlling a flow rate of a carrier gas sent from the carrier gas source 62 b, a mist generation source 64 that stores a raw material solution 64 a, a vessel 65 that contains water 65 a, an ultrasonic vibrator 66 attached to a bottom surface of the vessel 65, a quartz tube 67 serving as a film forming chamber, a heater 68 installed around the quartz tube 67, and an exhaust port 71. The susceptor 70 is composed of quartz, and the surface on which the substrate 69 is placed is inclined from the horizontal plane.

The raw material solution 64 a used in the mist CVD method is not limited as long as a semiconductor layer composed of a nitride or oxide of Group 13 elements can be obtained, and examples thereof include an organometallic complex or halide of Ga and/or a metal forming a solid solution with Ga is dissolved in a solvent. Examples of organometallic complexes include acetylacetonate complexes. Further in a case where a dopant is added to the semiconductor layer, a solution of a dopant component may be added to the raw material solution. Furthermore, an additive such as hydrochloric acid may be added to the raw material solution. Water, alcohol, or the like can be used as the solvent.

Next, the obtained raw material solution 64 a is atomized or formed into droplets to generate mist or droplet 64 b. A preferable example of the atomizing or droplet forming method is a method of vibrating the raw material solution 64 a using the ultrasonic vibrator 66. Then, the obtained mist or droplet 64 b is transported to the film forming chamber using a carrier gas. The carrier gas is not particularly limited, but one or more of an inert gas such as oxygen, ozone and nitrogen, and a reducing gas such as hydrogen can be used.

The substrate 69 is provided in the film forming chamber (quartz tube 67). The mist or droplet 64 b transported to the film forming chamber is thermally decomposed and chemically reacted there to form a film on the substrate 69. The reaction temperature varies depending on the kind of the raw material solution, but is preferably 300 to 800° C., and more preferably 400 to 700° C. Further, the atmosphere in the film forming chamber is not particularly limited as long as a desired semiconductor film can be obtained, and may be an oxygen gas atmosphere, an inert gas atmosphere, a vacuum or a reducing atmosphere, but an air atmosphere is preferable.

The semiconductor layer thus prepared using the base substrate typically has a significantly low surface crystal defect density of 1.0×10⁶/cm² or less. Such a semiconductor layer having an extremely low crystal defect density is excellent in dielectric breakdown electric field characteristics and is suitable for application in power semiconductors. The crystal defect density of the semiconductor layer can be evaluated by plane TEM observation (plan view) or cross-sectional TEM observation using a general transmission electron microscope (TEM). For example, in the case of observing a plan view using H-90001UHR-I manufactured by Hitachi as the transmission electron microscope, the acceleration voltage may be 300 kV. The test piece may be cut out so as to include the film surface and processed by ion milling so that the measurement field of view is 50 μm×50 μm and the thickness of the test piece around the measurement field of view is 150 nm. By preparing 10 such test pieces and observing a TEM image with a total of 10 fields of view, the crystal defect density can be evaluated with high accuracy. The crystal defect density is preferably 1.0×10⁵/cm² or less, more preferably 4.0×10³/cm² or less, and there is no particular lower limit. Further, the crystal defect density can also be evaluated with high accuracy by processing by ion milling such that eight or more regions having a measurement field of view of 4.1 μm×3.1 μm can be observed, and the thickness around the measurement field of view is 150 nm, and observing the eight or more regions having a measurement field of view of 4.1 μm×3.1 μm. The crystal defect density is preferably 1.0×10⁷/cm² or less, more preferably 1.0×10⁶/cm² or less, still more preferably 4.0×10³/cm² or less, and there is no particular lower limit.

As far as the present inventor knows, a technique for obtaining a semiconductor layer having such a low crystal defect density has not been conventionally known. For example, Non-Patent Literature 1 discloses that an α-Ga₂O₃ layer is formed using a substrate in which a (Al_(x), Ga_(1-x))₂O₃ layer (x=0.2 to 0.9) is introduced as a buffer layer between sapphire and the α-Ga₂O₃ layer, and in the obtained α-Ga₂O₃ layer, the densities of edge dislocations and screw dislocations are 3×10⁸/cm² and 6×10⁸/cm², respectively.

EXAMPLES

The present invention will be described in more detail with reference to the following examples.

Example 1

(1) Preparation of Composite Base Substrate

(1a) Preparation of Orientation Precursor Layer

An AD film (orientation precursor layer) composed of Cr₂O₃ was formed on a seed substrate (sapphire substrate) by an aerosol deposition (AD) apparatus 20 shown in FIG. 1 using a powder obtained by mixing a commercially available Cr₂O₃ powder and a commercially available Fe₂O₃ powder at a molar ratio of 72:28 as the raw material powder and sapphire (diameter 50.8 mm (2 inches), thickness 0.43 mm, c-plane, off-angle 0.3°) as the substrate. The configuration of the aerosol deposition (AD) apparatus 20 is as described above.

The AD film formation conditions were as follows. That is, Ar was used as a carrier gas, and a ceramic nozzle having a slit having a long side of 5 mm and a short side of 0.3 mm was used. The scanning conditions of the nozzle were to move 55 mm in the direction perpendicular to the long side of the slit and forward, to move 5 mm in the long side direction of the slit, to move 55 mm in the direction perpendicular to the long side of the slit and backward, and to move 5 mm in the long side direction of the slit and opposite to the initial position, repeatedly at a scanning speed of 0.5 mm/s, and at the time of 55 mm movement from the initial position in the long side direction of the slit, scanning was performed in the direction opposite to the previous direction, and the nozzle returned to the initial position. This was defined as one cycle, and repeated for 500 cycles. In one cycle of film formation at room temperature, the set pressure of the transport gas was adjusted to 0.07 MPa, the flow rate was adjusted to 8 L/min, and the pressure in the chamber was adjusted to 100 Pa or less. The AD film (orientation precursor layer) thus formed had a thickness of 120 μm.

(1b) Heat Treatment of Orientation Precursor Layer

The sapphire substrate on which the AD film (orientation precursor layer) was formed was taken out from the AD apparatus and annealed at 1700° C. for 4 hours in a nitrogen atmosphere.

(1c) Grinding and Polishing

The obtained substrate was fixed to a ceramic surface plate, the surface on the side derived from the AD film was ground using a grinding stone having a grit size of #2000 or less until the orientation layer was exposed, and then the plate surface was further smoothed by lapping using diamond abrasive grains. At this time, lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. Thereafter, mirror finishing was performed by chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate having an orientation layer on a sapphire substrate. The arithmetical mean roughness Ra of the orientation layer front surfaces after processing was 0.1 nm, the amount of grinding and polishing was 70 μm, and the thicknesses of the composite base substrate after polishing was 0.48 mm. The surface on the side on which the AD film is formed is referred to as a “front surface”.

(2) Evaluation of Orientation Layer

(2a) Cross-Section EDX

The composition of the cross-section orthogonal to the main surface of the substrate was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr, Fe, and O were detected in the range from the front surface of the composite base substrate to a depth of 20 μm. It was found that the ratio of Cr, Fe, and O did not substantially change in the range of the depth of 20 μm, and a Cr—Fe oxide layer (composition stable region) having a thickness of 20 μm was formed. Further, Cr, Fe, O, and Al were detected in the range from the Cr—Fe oxide layer to a depth of further 60 μm, and it was found that a Cr—Fe—Al oxide layer (gradient composition layer) having a thickness of 60 μm was formed between the Cr—Fe oxide layer and the sapphire substrate. In the Cr—Fe—Al oxide layer, the ratios of Al to Cr and Fe were different, and it was observed that the Al concentration was high on the sapphire substrate side and decreased on the side close to the Cr—Fe oxide layer. From the above, it was found that the thickness of the composition stable region was 20 μm, the thickness of the gradient composition layer was 60 μm, and the thickness of the entire orientation layer was 80 μm. It was shown that the thickness of the composite base substrate was 0.48 mm, and the thickness of the sapphire substrate in the composite base substrate was 0.40 mm.

(2b) Surface EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the surface of the orientation layer composed of the Cr oxide layer in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement were as follows.

<EBSD Measurement Conditions>

Acceleration voltage: 15 kV

Spot intensity: 70

Working distance: 22.5 mm

Step size: 0.5 μm

Sample tilt angle: 70°

Measurement program: Aztec (version 3.3)

From the obtained reverse pole figure orientation mapping, it was found that the Cr—Fe oxide layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Fe oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that the orientation layer composed of a solid solution of α-Cr₂O₃ and α-Fe₂O₃ was formed on the substrate front surface. Based on the above results, the preparation step of the composite base substrate is schematically shown in FIGS. 3(a) to 3(d).

(2 c) XRD

XRD in-plane measurement of the substrate front surface was performed using a multifunctional high-resolution X-ray diffractometer (D8 DISCOVER, manufactured by Bruker AXS Inc.). Specifically, after the Z axis was adjusted in accordance with the height of the substrate front surface, the axis was set by adjusting χ, φ, ω, and 2θ with respect to the (11-20) plane of the corundum-type crystal structure, and 2θ-ω measurement was performed under the following conditions.

<XRD Measurement Conditions>

Tube voltage: 40 kV

Tube current: 40 mA

Detector: Triple Ge (220) Analyzer

CuKα rays converted to parallel monochromatic light (half width 28 seconds) with a Ge (022) asymmetric reflection monochromator.

Step width: 0.001°

Scan speed: 1.0 second/step

As a result, it was found that the a-axis length of the front surface of the orientation layer was 4.982 Å.

(2d) Evaluation of Rear Surface of Orientation Layer (Sapphire Substrate Side)

A composite base substrate was separately prepared in the same manner as in (1) above. The front surface (orientation layer side) of the obtained composite base substrate was adhered to another sapphire substrate, and the sapphire substrate having a thickness of 0.40 mm disposed on the rear surface side of the composite base substrate was removed by grinding to expose the rear surface of the orientation layer. Next, the surface from which the sapphire substrate had been removed (orientation layer rear surface) was ground to #2000 using a grinding stone to flatten the plate surface. Next, the plate surface was smoothed by lapping using diamond abrasive grains. At this time, lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 pm to 0.5 μm, thereby improving the flatness of the plate surface. Thereafter, mirror finishing was performed by chemical mechanical polishing (CMP) using colloidal silica to prepare a sample for evaluating the rear surface of the orientation layer. From the above, the preparation step of the sample for evaluating the rear surface of the orientation layer are schematically shown in FIGS. 4(a) to 4(c).

EBSD measurement of the rear surface of the orientation layer was performed in the same manner as in (2b) above. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Fe—Al oxide layer constituting the rear surface of the orientation layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Fe—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. Since the rear surface of the orientation layer belongs to the composition gradient layer, it was found that the composition gradient layer is composed of a solid solution of Cr₂O₃, Fe₂O₃, and Al₂O₃.

Next, XRD in-plane measurement of the rear surface of the orientation layer was performed in the same manner as in (2c) above. As a result, it was found that the rear surface of the orientation layer was also attributed to the biaxially oriented single-phase corundum material, and the a-axis length was 4.762 Å. From this, it was shown that the a-axis length of the front surface of the orientation layer is longer than the a-axis length of the rear surface of the orientation layer (that is, [{(a-axis length of front surface of orientation layer)−(a-axis length of rear surface of orientation layer)}/(a-axis length of rear surface of orientation layer)]×100=4.6%).

(3) Formation of α-Ga₂O₃ film by mist CVD method

An α-Ga₂O₃ film was formed on a front surface of the orientation layer of the obtained composite base substrate as follows using the mist CVD apparatus 61 shown in FIG. 2.

(3a) Preparation of Raw Material Solution

An aqueous solution having a gallium acetylacetonate concentration of 0.05 mol/L was prepared. At this time, 38% hydrochloric acid was contained in a volume ratio of 1.8% to prepare a raw material solution 64 a.

(3b) Preparation for Film Formation

The obtained raw material solution 64 a was stored in the mist generation source 64. The composite base substrate prepared in (1) above was placed on the susceptor 70 as the substrate 69, and the heater 68 was operated to raise the temperature inside the quartz tube 67 to 610° C. Next, the flow control valves 63 a and 63 b were opened to supply the dilution gas and the carrier gas from the dilution gas source 62 a and the carrier gas source 62 b, respectively, into the quartz tube 67, and after the atmosphere of the quartz tube 67 was sufficiently replaced with the dilution gas and the carrier gas, the flow rates of the dilution gas and the carrier gas were adjusted to 0.6 L/min and 1.2 L/min, respectively. Nitrogen gas was used as the dilution gas and the carrier gas.

(3c) Film Formation

The ultrasonic vibrator 66 was vibrated at 2.4 MHz, and the vibration was propagated to the raw material solution 64 a through the water 65 a to mist the raw material solution 64 a and generate mist 64 b. The mist 64 b was introduced into the quartz tube 67 as a film forming chamber by the dilution gas and the carrier gas, and reacted in the quartz tube 67 to form a film on the substrate 69 by the CVD reaction on the surface of the substrate 69. Thus, a crystalline semiconductor film (semiconductor layer) was obtained. The film formation time was 60 minutes.

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the surface of the film composed of the Ga oxide in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement were as follows.

<EBSD Measurement Conditions>

Acceleration voltage: 15 kV

Spot intensity: 70

Working distance: 22.5 mm

Step size: 0.5 μm

Sample tilt angle: 70°

Measurement program: Aztec (version 3.3)

From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

Plane TEM observation (plan view) was performed to evaluate the crystal defect density of the α-Ga₂O₃ film. The test piece was cut out so as to include the surface on the film formation side, and processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained sections were subjected to TEM observation at an acceleration voltage of 300 kV using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) to evaluate the crystal defect density. Actually, eight TEM images having a measurement field of view of 4.1 μm×3.1 μm were observed. As a result, no crystal defect was observed in the obtained TEM images, and the crystal defect density was found to be at least less than 9.9×10⁵/cm².

Example 2

(1) Preparation of Composite Base Substrate

A composite base substrate was prepared in the same manner as in Example 1 (1) except that a powder obtained by mixing a commercially available Cr₂O₃ powder, a commercially available Fe₂O₃ powder, and a commercially available Al₂O₃ powder at a molar ratio of 45:45:10 was used as the raw material powder for the AD film.

(2) Evaluation of Orientation Layer

(2a) Cross-Section EDX

The composition of the cross-section orthogonal to the main surface of the substrate was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr, Fe, Al, and O were detected in the range from the front surface of the composite base substrate to a depth of 20 μm. It was found that the ratio of Cr, Fe, Al, and O did not substantially change in the range up to the depth of 20 μm, and a Cr—Fe—Al oxide layer (composition stable region) having a thickness of 20 μm was formed. Further, Cr, Fe, O and Al were also detected in the range from the Cr—Fe—Al oxide layer to a depth of further 60 μm, but in this region, the ratio of Al to Cr and Fe was different, and it was observed that the Al concentration was high on the sapphire substrate side and decreased on the side close to the composition stable region. Therefore, it was found that this range is a composition gradient region. From this, it was found that the thickness of the composition stable region was 20 μm, the thickness of the gradient composition layer was 60 μm, and the thickness of the entire orientation layer was 80 μm. It was shown that the thickness of the composite base substrate was 0.48 mm, and the thickness of the sapphire substrate in the composite base substrate was 0.40 mm.

(2b) Surface EBSD

In the same manner as in Example 1 (2b), the reverse pole figure orientation mapping of the substrate front surface composed of the Cr—Fe—Al oxide layer was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Fe—Al oxide layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Fe—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that the orientation layer composed of a solid solution of α-Cr₂O₃, α-Fe₂O₃, and α-Al₂O₃ was formed on the substrate front surface.

(2c) XRD

XRD in-plane measurement of the substrate front surface was performed in the same manner as in Example 1 (2c). As a result, it was found that the a-axis length of the front surface of the orientation layer was 4.975 Å.

(2d) Evaluation of Rear Surface of Orientation Layer (Sapphire Substrate Side)

A composite base substrate was separately prepared in the same manner as in the above (1), and then a sample for evaluating the rear surface of the orientation layer was prepared in the same manner as in Example 1 (2d). EBSD measurement of the rear surface of the orientation layer was performed in the same manner as in (2b) above. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Fe—Al oxide layer constituting the rear surface of the orientation layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Fe—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. Since the rear surface of the orientation layer belongs to the composition gradient layer, it was found that the composition gradient layer is composed of a solid solution of Cr₂O₃, Fe₂O₃, and Al₂O₃. Further, XRD in-plane measurement of the rear surface of the orientation layer was performed in the same manner as in (2c) above. As a result, it was found that the rear surface of the orientation layer was also attributed to the biaxially oriented single-phase corundum material, and the a-axis length was 4.755 Å. From this, it was shown that the a-axis length of the front surface of the orientation layer is longer than the a-axis length of the rear surface of the orientation layer (that is, [{(a-axis length of front surface of orientation layer)−(a-axis length of rear surface of orientation layer)}/(a-axis length of rear surface of orientation layer)]×100=4.6%).

(3) Formation of α-Ga₂O₃ Film by Mist CVD Method

An α-Ga₂O₃ film was formed on the composite base substrate in the same manner as in Example 1 (3).

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

In the same manner as in Example 1 (4b), the reverse pole figure orientation mapping of the film surface composed of the Ga oxide was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

The crystal defect density on film forming side surface of the α-Ga₂O₃ film was evaluated in the same manner as in Example 1 (4c). As a result, no crystal defect was observed in the obtained TEM images, and the crystal defect density was found to be at least less than 9.9×10⁵/cm².

Example 3

(1) Preparation of Base Substrate

(1a) Preparation of Composite Base Substrate

A composite base substrate was prepared in the same manner as in Example 1 (1).

(1b) Thickening of Orientation Layer

In order to thicken the orientation layer, the AD film (orientation precursor layer) was formed again on the orientation layer of the composite base substrate. The AD film forming apparatus 20 shown in FIG. 1 formed an AD film (orientation precursor layer) composed of Cr₂O₃ and Fe₂O₃ on the orientation layer of the composite base substrate.

The AD film formation conditions were as follows. That is, Ar was used as a carrier gas, and a ceramic nozzle having a slit having a long side of 5 mm and a short side of 0.3 mm was used. The scanning conditions of the nozzle were to move 55 mm in the direction perpendicular to the long side of the slit and forward, to move 5 mm in the long side direction of the slit, to move 55 mm in the direction perpendicular to the long side of the slit and backward, and to move 5 mm in the long side direction of the slit and opposite to the initial position, repeatedly at a scanning speed of 0.5 mm/s, at the time of 55 mm movement from the initial position in the long side direction of the slit, scanning was performed in the direction opposite to the previous direction, and the nozzle returned to the initial position. This was defined as one cycle, and repeated for 500 cycles. In one cycle of film formation at room temperature, the set pressure of the transport gas was adjusted to 0.07 MPa, the flow rate was adjusted to 8 L/min, and the pressure in the chamber was adjusted to 100 Pa or less. The AD film thus formed had a thickness of 120 μm.

The composite base substrate on which the AD film was formed was taken out from the AD apparatus and annealed at 1700° C. for 4 hours in a nitrogen atmosphere.

The obtained substrate was fixed to a ceramic surface plate, the surface on the side derived from the AD film was ground using a grinding stone having a grit size of #2000 or less until the orientation layer was exposed, and then the plate surface was further smoothed by lapping using diamond abrasive grains. At this time, lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. Thereafter, mirror finishing was performed by chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate having an orientation layer on a sapphire substrate. The arithmetical mean roughness Ra of the orientation layer front surfaces after processing was 0.1 nm, the amount of grinding and polishing was 50 pm, and the thicknesses of the substrate after completion of polishing was 0.48 mm.

The series of steps for increasing the thickness (that is, AD film formation-anneal-grinding and polishing) was repeated nine times in total (that is, the series of steps was performed ten times in total when the procedure for preparing the composite base substrate of (1a) was counted as one time). As a result, the thickness of the composite base substrate after the final polishing was completed was 0.93 mm. The surface on the side on which the AD film is formed is referred to as a “front surface”.

(1c) Self-Standing of Orientation Layer

The substrate thus obtained was fixed to a ceramic surface plate, and the surface (rear surface) opposite to the surface (front surface) derived from the AD film, that is, the surface on the sapphire substrate side was ground using a grinding stone to remove the sapphire substrate. Thereafter, the front surface of the orientation layer on the side from which the sapphire substrate had been removed was ground to #2000 using a grinding stone to flatten the plate surface. Next, the plate surface was smoothed by lapping using diamond abrasive grains. At this time, lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. The amount of grinding and polishing including sapphire was 480 μm, and the thickness of the substrate after completion of polishing was 0.45 mm. The obtained substrate became a self-standing substrate composed of only the orientation layer.

(2) Evaluation of Self-Standing Substrate

(2a) Cross-Section EDX

The composition of the cross-section orthogonal to the main surface of the substrate was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr, Fe, and O were detected in the entire cross-section of the self-standing substrate. The ratios of Cr, Fe, and O remained almost unchanged over the entire region, and it was found that the self-standing substrate was composed of a single-phase Cr—Fe oxide.

(2b) Surface EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the front surface (surface on which the AD film was formed) and rear surface (surface on which the sapphire substrate was in contact) of the substrate composed of the Cr oxide layer in a field of view of 500 μm×500 μm. The conditions for the EBSD measurement were the same as in Example 1 (2b). From the obtained reverse pole figure orientation mapping, it was found that the self-standing substrate was composed of a biaxially oriented corundum-type crystal structure in which the self-standing substrate was c-axis oriented in the substrate normal direction on both the front surface and the rear surface and was also oriented in the in-plane direction. From these, it was shown that the front surface and the rear surface of the self-standing substrate were composed of a solid solution of α-Cr₂O₃ and α-Fe₂O₃ oriented biaxially.

(2c) XRD

The XRD in-plane measurement of the front surface and the rear surface of the self-standing substrate was performed in the same manner as in Example 1 (2c). As a result, it was found that the a-axis lengths of the front surface and the rear surface of the self-standing substrate were both 4.982 Å.

(3) Formation of α-Ga₂O₃ film by mist CVD method

An α-Ga₂O₃ film was formed on the self-standing substrate in the same manner as in Example 1 (3).

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

In the same manner as in Example 1 (4b), the reverse pole figure orientation mapping of the film surface composed of the Ga oxide was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

The crystal defect density on film forming side surface of the α-Ga₂O₃ film was evaluated in the same manner as in Example 1 (4c). As a result, no crystal defect was observed in the obtained TEM images, and the crystal defect density was found to be at least less than 9.9×10⁵/cm².

Example 4

(1) Preparation of Composite Base Substrate

(1a) Preparation of Orientation Precursor Layer

Using a commercially available Cr₂O₃ powder as the raw material powder, an AD film (orientation precursor layer) composed of Cr₂O₃ was formed on a seed substrate (sapphire substrate) by the AD apparatus 20 shown in FIG. 1.

The AD film formation conditions were as follows. That is, Ar was used as a carrier gas, and a ceramic nozzle having a slit having a long side of 5 mm and a short side of 0.3 mm was used. The scanning conditions of the nozzle were to move 55 mm in the direction perpendicular to the long side of the slit and forward, to move 5 mm in the long side direction of the slit, to move 55 mm in the direction perpendicular to the long side of the slit and backward, and to move 5 mm in the long side direction of the slit and opposite to the initial position, repeatedly at a scanning speed of 0.5 mm/s, and at the time of 55 mm movement from the initial position in the long side direction of the slit, scanning was performed in the direction opposite to the previous direction, and the nozzle returned to the initial position.

This was defined as one cycle, and repeated for 500 cycles. In one cycle of film formation at room temperature, the set pressure of the transport gas was adjusted to 0.07 MPa, the flow rate was adjusted to 8 L/min, and the pressure in the chamber was adjusted to 100 Pa or less. The AD film thus formed had a thickness of 120 μm.

(1b) Heat Treatment of Orientation Precursor Layer

The sapphire substrate on which the AD film was formed was taken out from the AD apparatus and annealed at 1700° C. for 4 hours in a nitrogen atmosphere.

(1c) Grinding and Polishing

The obtained substrate was fixed to a ceramic surface plate, the surface on the side derived from the AD film was ground using a grinding stone having a grit size of #2000 or less until the orientation layer was exposed, and then the plate surface was further smoothed by lapping using diamond abrasive grains. At this time, lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 pm to 0.5 μm, thereby improving the flatness of the plate surface. Thereafter, mirror finishing was performed by chemical mechanical polishing (CMP) using colloidal silica to obtain a composite base substrate having an orientation layer on a sapphire substrate. The arithmetical mean roughness Ra of the orientation layer front surfaces after processing was 0.1 nm, the amount of grinding and polishing was 100 μm, and the thicknesses of the composite base substrate after completion of polishing was 0.45 mm. The surface on the side on which the AD film is formed is referred to as a “front surface”.

(2) Evaluation of Orientation Layer

(2a) Cross-Section EDX

The composition of the cross-section orthogonal to the main surface of the substrate was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, Cr, O and Al were detected in the range from the front surface of the composite base substrate to a depth of 50 μm, and it was found that a Cr-Al oxide layer (gradient composition layer) having a thickness of 50 μm was formed between the front surface and the sapphire substrate. In the Cr—Al oxide layer, the ratios of Cr and Al were different, and it was observed that the Al concentration was high on the sapphire substrate side and decreased on the side close to the front surface. From the above, it was found that the thickness of the entire orientation layer was 50 μm. It was shown that the thickness of the composite base substrate was 0.45 mm, and the thickness of the sapphire substrate in the composite base substrate was 0.40 mm.

(2b) Surface EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the surface of the substrate composed of the Cr—Al oxide layer in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement were as follows.

<EBSD Measurement Conditions>

Acceleration voltage: 15 kV

Spot intensity: 70

Working distance: 22.5 mm

Step size: 0.5 μm

Sample tilt angle: 70°

Measurement program: Aztec (version 3.3)

From the obtained reverse pole figure orientation mapping, it was found that the Cr—Al oxide layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that the orientation layer composed of a solid solution of α-Cr₂O₃ and α-Al₂O₃ was formed on the substrate front surface.

(2c) XRD

XRD in-plane measurement of the substrate front surface was performed using a multifunctional high-resolution X-ray diffractometer (D8 DISCOVER, manufactured by Bruker AXS Inc.). Specifically, after the Z axis was adjusted in accordance with the height of the substrate front surface, the axis was set by adjusting χ, φ, ω, and 2θ with respect to the (11-20) plane, and 2θ-ω measurement was performed under the following conditions.

<XRD Measurement Conditions>

Tube voltage: 40 kV

Tube current: 40 mA

Detector: Triple Ge (220) Analyzer

CuKα rays converted to parallel monochromatic light (half width 28 seconds) with a Ge (022) asymmetric reflection monochromator.

Step width: 0.001°

Scan speed: 1.0 second/step

As a result, it was found that the a-axis length of the front surface of the orientation layer was 4.892 Å.

(2d) Evaluation of Rear Surface of Orientation Layer (Sapphire Substrate Side)

A composite base substrate was separately prepared in the same manner as in (1) above. The front surface (orientation layer side) of the obtained composite base substrate was adhered to another sapphire substrate, and the sapphire substrate having a thickness of 0.4 mm disposed on the rear surface side of the composite base substrate was removed by grinding to expose the rear surface of the orientation layer. Next, the surface from which the sapphire substrate had been removed (orientation layer rear surface) was ground to #2000 using a grinding stone to flatten the plate surface. Next, the plate surface was smoothed by lapping using diamond abrasive grains. At this time, lapping was performed while gradually reducing the size of the diamond abrasive grains from 3 μm to 0.5 μm, thereby improving the flatness of the plate surface. Thereafter, mirror finishing was performed by chemical mechanical polishing (CMP) using colloidal silica to prepare a sample for evaluating the rear surface of the orientation layer.

EBSD measurement of the rear surface of the orientation layer was performed in the same manner as in (2b) above. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Al oxide layer constituting the rear surface of the orientation layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. Since the rear surface of the orientation layer belongs to the composition gradient layer, it was found that the composition gradient layer is composed of a solid solution of Cr₂O₃ and Al₂O₃.

Next, XRD in-plane measurement of the rear surface of the orientation layer was performed in the same manner as in (2c) above. As a result, it was found that the rear surface of the orientation layer was also attributed to the biaxially oriented single-phase corundum material, and the a-axis length was 4.756 Å. From this, it was shown that the a-axis length of the front surface of the orientation layer is longer than the a-axis length of the rear surface of the orientation layer, and the difference between the a-axis lengths of the front surface and the rear surface (=[{(a-axis length of front surface)−(a-axis length of rear surface)}/(a-axis length of rear surface)]×100) is 2.9%.

(3) Formation of α-Ga₂O₃ Film by Mist CVD Method

An α-Ga₂O₃ film was formed on a front surface of the orientation layer of the obtained composite base substrate as follows using the mist CVD apparatus 61 shown in FIG. 2.

(3 a) Preparation of Raw Material Solution

An aqueous solution having a gallium acetylacetonate concentration of 0.05 mol/L was prepared. At this time, 38% hydrochloric acid was contained in a volume ratio of 1.8% to prepare a raw material solution 64 a.

(3b) Preparation for Film Formation

The obtained raw material solution 64 a was stored in the mist generation source 64. The composite base substrate prepared in (1) above was placed on the susceptor 70 as the substrate 69, and the heater 68 was operated to raise the temperature inside the quartz tube 67 to 610° C. Next, the flow control valves 63 a and 63 b were opened to supply the dilution gas and the carrier gas from the dilution gas source 62 a and the carrier gas source 62 b, respectively, into the quartz tube 67, and after the atmosphere of the quartz tube 67 was sufficiently replaced with the dilution gas and the carrier gas, the flow rates of the dilution gas and the carrier gas were adjusted to 0.6 L/min and 1.2 L/min, respectively. Nitrogen gas was used as the dilution gas and the carrier gas.

(3c) Film Formation

The ultrasonic vibrator 66 was vibrated at 2.4 MHz, and the vibration was propagated to the raw material solution 64 a through the water 65 a to mist the raw material solution 64 a and generate mist 64 b. The mist 64 b was introduced into the quartz tube 67 as a film forming chamber by the dilution gas and the carrier gas, and reacted in the quartz tube 67 to form a film on the substrate 69 by the CVD reaction on the surface of the substrate 69. Thus, a crystalline semiconductor film (semiconductor layer) was obtained. The film formation time was 60 minutes.

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the surface of the film composed of the Ga oxide in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement were as follows.

<EBSD Measurement Conditions>

Acceleration voltage: 15 kV

Spot intensity: 70

Working distance: 22.5 mm

Step size: 0.5 μm

Sample tilt angle: 70°

Measurement program: Aztec (version 3.3)

From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

Plane TEM observation (plan view) was performed to evaluate the crystal defect density of the α-Ga₂O₃ film. The test piece was cut out so as to include the surface on the film formation side, and processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained sections were subjected to TEM observation at an acceleration voltage of 300 kV using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) to evaluate the crystal defect density. Actually, eight TEM images having a measurement field of view of 4.1 μm×3.1 μm were observed. As a result, it was found from the number of crystal defects in the obtained TEM image that the crystal defect density was 9.4×10⁶/cm².

Example 5

(1) Preparation of Composite Base Substrate

A composite base substrate was prepared in the same manner as in Example 1 (1) except that a powder obtained by mixing a commercially available Cr₂O₃ powder and a commercially available TiO₂ powder at a molar ratio of 97:3 was used as the raw material powder for the AD film.

(2) Evaluation of Orientation Layer

(2a) Cross-Section EDX

The composition of the cross-section orthogonal to the main surface of the substrate was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr, Ti, and O were detected in the range from the front surface of the composite base substrate to a depth of 20 μm. It was found that the ratio of Cr, Ti, and O did not substantially change in the range of 20 μm, and a Cr—Ti oxide layer (composition stable region) having a thickness of 20 μm was formed. Further, Cr, Ti, O, and Al were detected in the range from the Cr—Ti oxide layer to a depth of further 60 μm, and it was found that a Cr—Ti—Al oxide layer (gradient composition layer) having a thickness of 60 μm was formed between the Cr—Ti oxide layer and the sapphire substrate. In the Cr—Ti—Al oxide layer, the ratios of Al to Cr and Ti were different, and it was observed that the Al concentration was high on the sapphire substrate side and decreased on the side close to the Cr—Ti oxide layer. From this, it was found that the thickness of the composition stable region was 20 μm, the thickness of the gradient composition layer was 60 μm, and the thickness of the entire orientation layer was 80 μm. It was shown that the thickness of the composite base substrate was 0.48 mm, and the thickness of the sapphire substrate in the composite base substrate was 0.40 mm.

(2b) Surface EBSD

In the same manner as in Example 1 (2b), the reverse pole figure orientation mapping of the substrate front surface composed of the Cr—Ti oxide layer was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Ti oxide layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Ti oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation layer composed of a material in which Ti component was dissolved in α-Cr₂O₃ was formed on the substrate front surface.

(2c) XRD

The XRD in-plane measurement of the substrate front surface was performed in the same manner as in Example 1 (2c). As a result, it was found that the a-axis length of the front surface of the orientation layer was 4.971 Å.

(2d) Evaluation of Rear Surface of Orientation Layer (Sapphire Substrate Side)

A composite base substrate was separately prepared in the same manner as in the above (1), and then a sample for evaluating the rear surface of the orientation layer was prepared in the same manner as in Example 1 (2d). EBSD measurement of the rear surface of the orientation layer was performed in the same manner as in (2b) above. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Ti—Al oxide layer constituting the rear surface of the orientation layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Ti—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. Since the rear surface of the orientation layer belongs to the composition gradient layer, it was found that the composition gradient layer is composed of a solid solution of Cr₂O₃, Ti component, and Al₂O₃. Further, XRD in-plane measurement of the rear surface of the orientation layer was performed in the same manner as in (2c) above. As a result, it was found that the rear surface of the orientation layer was also attributed to the biaxially oriented single-phase corundum material, and the a-axis length was 4.756 Å. From this, it was shown that the a-axis length of the front surface of the orientation layer is longer than the a-axis length of the rear surface of the orientation layer, and the difference between the a-axis lengths of the front surface and the rear surface (=[{(a-axis length of front surface)−(a-axis length of rear surface)}/(a-axis length of rear surface)]×100) is 4.5%.

(3) Formation of α-Ga₂O₃ Film by Mist CVD Method

An α-Ga₂O₃ film was formed on the composite base substrate in the same manner as in Example 1 (3).

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

In the same manner as in Example 1 (4b), the reverse pole figure orientation mapping of the film surface composed of the Ga oxide was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

Plane TEM observation (plan view) was performed to evaluate the crystal defect density of the α-Ga₂O₃ film. The test piece was cut out so as to include the surface on the film formation side, and processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained sections were subjected to TEM observation at an acceleration voltage of 300 kV using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) to evaluate the crystal defect density. Actually, eight TEM images having a measurement field of view of 4.1 μm×3.1 μm were observed. As a result, no crystal defect was observed in the obtained TEM images, and the crystal defect density was found to be at least less than 9.9×10⁵/cm².

Example 6 (1) Preparation of Composite Base Substrate

A composite base substrate was prepared in the same manner as in Example 1 (1) except that a powder obtained by mixing a commercially available Cr₂O₃ powder, a commercially available TiO₂ powder, and a commercially available Fe₂O₃ powder at a molar ratio of 82:2:16 was used as the raw material powder for the AD film.

(2) Evaluation of Orientation Layer

(2a) Cross-Section EDX

The composition of the cross-section orthogonal to the main surface of the substrate was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr, Ti, Fe, and O were detected in the range from the front surface of the composite base substrate to a depth of 20 μm. It was found that the ratio of Cr, Ti, Fe, and O did not substantially change in the range of 20 μm, and a Cr—Ti—Fe oxide layer (composition stable region) having a thickness of 20 μm was formed. Further, Cr, Ti, Fe, O, and Al were detected in the range from the Cr—Ti—Fe oxide layer to a depth of further 60 μm, and it was found that a Cr—Ti—Fe—Al oxide layer (gradient composition layer) having a thickness of 60 μm was formed between the Cr—Ti—Fe oxide layer and the sapphire substrate. In the Cr—Ti—Fe—Al oxide layer, the ratios of Al to Cr, Ti, and Fe were different, and it was observed that the Al concentration was high on the sapphire substrate side and decreased on the side close to the Cr—Ti—Fe oxide layer. From the above, it was found that the thickness of the composition stable region was 20 μm, the thickness of the gradient composition layer was 60 μm, and the thickness of the entire orientation layer was 80 μm. It was shown that the thickness of the composite base substrate was 0.48 mm, and the thickness of the sapphire substrate in the composite base substrate was 0.40 mm.

(2b) Surface EBSD

In the same manner as in Example 1 (2b), the reverse pole figure orientation mapping of the substrate front surface composed of the Cr—Ti—Fe oxide layer was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Ti—Fe oxide layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Ti—Fe oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that the orientation layer composed of a solid solution of α-Cr₂O₃, Ti component, and α-Fe₂O₃ was formed on the substrate front surface.

(2c) XRD

The XRD in-plane measurement of the substrate front surface was performed in the same manner as in Example 1 (2c). As a result, it was found that the a-axis length of the front surface of the orientation layer was 4.981 Å.

(2d) Evaluation of Rear Surface of Orientation Layer (Sapphire Substrate Side)

A composite base substrate was separately prepared in the same manner as in the above (1), and then a sample for evaluating the rear surface of the orientation layer was prepared in the same manner as in Example 1 (2d). EBSD measurement of the rear surface of the orientation layer was performed in the same manner as in (2b) above. From the obtained reverse pole figure orientation mapping, it was found that the Cr—Ti—Fe—Al oxide layer constituting the rear surface of the orientation layer was a layer having a biaxially oriented corundum-type crystal structure in which the Cr—Ti—Fe—Al oxide layer was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. Since the rear surface of the orientation layer belongs to the composition gradient layer, it was found that the composition gradient layer is composed of a solid solution of Cr₂O_(e), Ti component, Fe₂O₃, and Al₂O₃. Further, XRD in-plane measurement of the rear surface of the orientation layer was performed in the same manner as in (2c) above. As a result, it was found that the rear surface of the orientation layer was also attributed to the biaxially oriented single-phase corundum material, and the a-axis length was 4.763 Å. From this, it was shown that the a-axis length of the front surface of the orientation layer is longer than the a-axis length of the rear surface of the orientation layer, and the difference between the a-axis lengths of the front surface and the rear surface (=[{(a-axis length of front surface)−(a-axis length of rear surface)}/(a-axis length of rear surface)]×100) is 4.6%.

(3) Formation of α-Ga₂O₃ Film by Mist CVD Method

An α-Ga₂O₃ film was formed on the composite base substrate in the same manner as in Example 1 (3).

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

In the same manner as in Example 1 (4b), the reverse pole figure orientation mapping of the film surface composed of the Ga oxide was performed in a field of view of 500 μm×500 μm. From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction and was also oriented in the in-plane direction. From these, it was shown that an orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

Plane TEM observation (plan view) was performed to evaluate the crystal defect density of the α-Ga₂O₃ film. The test piece was cut out so as to include the surface on the film formation side, and processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained sections were subjected to TEM observation at an acceleration voltage of 300 kV using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) to evaluate the crystal defect density. Actually, eight TEM images having a measurement field of view of 4.1 μm×3.1 μm were observed. As a result, no crystal defect was observed in the obtained TEM images, and the crystal defect density was found to be at least less than 9.9×10⁵/cm².

Example 7 (Comparison)

(1) Preparation of Base Substrate

Using the mist CVD apparatus 61 shown in FIG. 2, an α-Cr₂O₃ film was formed on the surface of a sapphire substrate (50.8 mm diameter (2 inches), 0.43 mm thickness, c-plane, 0.3° off-angle) as follows.

(1a) Preparation of Raw Material Solution

An aqueous solution having an ammonium dichromate concentration of 0.1 mol/L was prepared as the raw material solution 64 a.

(1b) Preparation for Film Formation

The obtained raw material solution 64 a was stored in the mist generation source 64. The sapphire substrate was placed on the susceptor 70 as the substrate 69, and the heater 68 was operated to raise the temperature inside the quartz tube 67 to 410° C. Next, the flow control valves 63 a and 63 b were opened to supply the dilution gas and the carrier gas from the dilution gas source 62 a and the carrier gas source 62 b, respectively, into the quartz tube 67, and after the atmosphere of the quartz tube 67 was sufficiently replaced with the dilution gas and the carrier gas, the flow rates of the dilution gas and the carrier gas were adjusted to 2.2 L/min and 4.8 L/min, respectively. Nitrogen gas was used as the dilution gas and the carrier gas.

(1c) Film Formation

The ultrasonic vibrator 66 was vibrated at 2.4 MHz, and the vibration was propagated to the raw material solution 64 a through the water 65 a to mist the raw material solution 64 a and generate mist 64 b. The mist 64 b was introduced into the quartz tube 67 as a film forming chamber by the dilution gas and the carrier gas, and reacted in the quartz tube 67 to form a film on the substrate 69 for 30 minutes by the CVD reaction on the surface of the substrate 69, thereby obtaining an oxide deposit layer.

(2) Evaluation of Orientation Layer

(2a) Surface EDX

The composition of the substrate front surface was analyzed using an energy dispersive X-ray analyzer (EDX). As a result, only Cr and O were detected, and it was found that the oxide deposit layer was Cr oxide. Note that it was difficult to perform cross-sectional EDX measurement due to the small film thickness.

(2b) Surface EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the surface of the substrate composed of the Cr oxide layer in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement were as follows.

<EBSD Measurement Conditions>

Acceleration voltage: 15 kV

Spot intensity: 70

Working distance: 22.5 mm

Step size: 0.5 μm

Sample tilt angle: 70°

Measurement program: Aztec (version 3.3)

From the obtained reverse pole figure orientation mapping, it was found that the Cr oxide layer is a layer having a biaxially oriented corundum-type crystal structure in which the Cr oxide layer was c-axis oriented in the substrate normal direction. However, a domain shifted by 60° was observed in the in-plane direction. From these, it was shown that a uniaxial orientation layer of α-Cr₂O₃ was formed on the substrate front surface.

(2c) XRD

XRD in-plane measurement of the substrate front surface was performed using a multifunctional high-resolution X-ray diffractometer (D8 DISCOVER, manufactured by Bruker AXS Inc.). Specifically, after the Z axis was adjusted in accordance with the height of the substrate front surface, the axis was set by adjusting χ, φ, ω, and 2θ with respect to the (11-20) plane) of the corundum-type crystal structure, and 2θ-ω measurement was performed under the following conditions.

<XRD Measurement Conditions>

Tube voltage: 40 kV

Tube current: 40 mA

Detector: Triple Ge (220) Analyzer

CuKα rays converted to parallel monochromatic light (half width 28 seconds) with a Ge (022) asymmetric reflection monochromator.

Step width: 0.001°

Scan speed: 1.0 second/step

As a result, it was found that the a-axis length of the front surface of the orientation layer was 4.961 Å.

(2d) Cross-Sectional TEM Observation of Orientation Layer

A cross-sectional TEM observation was performed to evaluate the change in the lattice constant of the α-Cr₂O₃ film. The cross-sectional sample was cut out so as to include the front surface and the rear surface (interface on the sapphire side) on the film formation side, and processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained sections were subjected to cross-sectional TEM observation at an acceleration voltage of 300 kV using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) to obtain electron-beam diffraction images of the front and rear surfaces of the a-Cr₂O3 film. The electron beam incident direction was Cr₂O₃ <1-10>. As a result, no difference was observed between the electron diffraction images of the front and rear surfaces of the film, and it was found that there was no change in lattice constants between the front and rear surfaces of the α-Cr₂O₃ film.

(3) Formation of α-Ga₂O₃ Film by Mist CVD Method

An α-Ga₂O₃ film was formed on a front surface of the obtained base substrate as follows using the mist CVD apparatus 61 shown in FIG. 2.

(3 a) Preparation of Raw Material Solution

An aqueous solution having a gallium acetylacetonate concentration of 0.05 mol/L was prepared. At this time, 38% hydrochloric acid was contained in a volume ratio of 1.8% to prepare a raw material solution 64 a.

(3b) Preparation for Film Formation

The obtained raw material solution 64 a was stored in the mist generation source 64. The composite base substrate prepared in (1) above was placed on the susceptor 70 as the substrate 69, and the heater 68 was operated to raise the temperature inside the quartz tube 67 to 610° C. Next, the flow control valves 63 a and 63 b were opened to supply the dilution gas and the carrier gas from the dilution gas source 62 a and the carrier gas source 62 b, respectively, into the quartz tube 67, and after the atmosphere of the quartz tube 67 was sufficiently replaced with the dilution gas and the carrier gas, the flow rates of the dilution gas and the carrier gas were adjusted to 0.6 L/min and 1.2 L/min, respectively. Nitrogen gas was used as the dilution gas and the carrier gas.

(3c) Film Formation

The ultrasonic vibrator 66 was vibrated at 2.4 MHz, and the vibration was propagated to the raw material solution 64 a through the water 65 a to mist the raw material solution 64 a and generate mist 64 b. The mist 64 b was introduced into the quartz tube 67 as a film forming chamber by the dilution gas and the carrier gas, and reacted in the quartz tube 67 to form a film on the substrate 69 by the CVD reaction on the surface of the substrate 69. Thus, a crystalline semiconductor film (semiconductor layer) was obtained. The film formation time was 60 minutes.

(4) Evaluation of Semiconductor Film

(4a) Surface EDS

As a result of EDS measurement of the surface of the obtained film, only Ga and O were detected, and it was found that the obtained film was a Ga oxide.

(4b) EBSD

An SEM (SU-5000, manufactured by Hitachi High-Technologies Corporation) equipped with an electron backscatter diffraction apparatus (EBSD) (Nordlys Nano, manufactured by Oxford Instruments Inc.) was used to perform reverse pole figure orientation mapping of the surface of the film composed of the Ga oxide in a field of view of 500 μm×500 μm. The conditions for this EBSD measurement were as follows.

<EBSD Measurement Conditions>

Acceleration voltage: 15 kV

Spot intensity: 70

Working distance: 22.5 mm

Step size: 0.5 μm

Sample tilt angle: 70°

Measurement program: Aztec (version 3.3)

From the obtained reverse pole figure orientation mapping, it was found that the Ga oxide film has a biaxially oriented corundum-type crystal structure in which the Ga oxide film was c-axis oriented in the substrate normal direction. However, a domain shifted by 60° was observed in the in-plane direction. From these, it was shown that a uniaxial orientation film composed of α-Ga₂O₃ was formed.

(4c) Plane TEM of Film Forming Side Surface

Plane TEM observation (plan view) was performed to evaluate the crystal defect density of the α-Ga₂O₃ film. The test piece was cut out so as to include the surface on the film formation side, and processed by ion milling so that the sample thickness (T) around the measurement field of view was 150 nm. The obtained sections were subjected to TEM observation at an acceleration voltage of 300 kV using a transmission electron microscope (H-90001UHR-I manufactured by Hitachi) to evaluate the crystal defect density. Actually, eight TEM images having a measurement field of view of 4.1 μm×3.1 μm were observed. As a result, a large number of crystal defects were observed in the obtained TEM images, and the crystal defect density was found to be at least 1.0×10¹¹/cm² or more. 

What is claimed is:
 1. A base substrate comprising an orientation layer used for crystal growth of a nitride or oxide of a Group 13 element, wherein a front surface of the orientation layer on a side used for the crystal growth is composed of a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire, wherein the orientation layer contains a solid solution containing two or more selected from the group consisting of α-Al₂O₃, α-Cr₂O₃, α-Fe₂O₃, α-Ti₂O₃, α-V₂O₃, and α-Rh₂O₃, and wherein a gradient composition region having a composition varying in a thickness direction is present in the orientation layer.
 2. The base substrate according to claim 1, wherein the base substrate is used for crystal growth of a semiconductor layer composed of α-Ga₂O₃ or an α-Ga₂O₃ solid solution, and the orientation layer is composed of a material containing a solid solution of α-Cr₂O₃ and a different material.
 3. The base substrate according to claim 1, wherein the a-axis length and/or the c-axis length on the front surface of the orientation layer is longer than an a-axis length and/or a c-axis length on a rear surface of the orientation layer by 2.5% or more.
 4. The base substrate according to claim 1, wherein the material having the corundum-type crystal structure contains a solid solution containing two or more selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, and α-Ti₂O₃, or a solid solution containing α-Al₂O₃ and one or more selected from the group consisting of α-Cr₂O₃, α-Fe₂O₃, and α-Ti₂O₃.
 5. The base substrate according to claim 1, wherein an entirety of the orientation layer is composed of the material having the corundum-type crystal structure.
 6. The base substrate according to claim 1, wherein the a-axis length of the material having the corundum-type crystal structure on the front surface is larger than 4.754 Å and 5.157 Å or less.
 7. The base substrate according to claim 6, wherein the a-axis length is 4.850 to 5.000 Å.
 8. The base substrate according to claim 1, wherein the orientation layer has a composition stable region located near the front surface and having a composition stable in a thickness direction, and a gradient composition region located far from the front surface and having a composition varying in the thickness direction.
 9. The base substrate according to claim 1, wherein the gradient composition region is composed of a solid solution containing α-Cr₂O₃ and α-Al₂O₃.
 10. The base substrate according to claim 1, wherein in the gradient composition region, an Al concentration decreases in the thickness direction toward the composition stable region.
 11. The base substrate according to claim 1, wherein the orientation layer is a heteroepitaxial growth layer.
 12. The base substrate according to claim 1, further comprising a support substrate on a side opposite to the front surface of the orientation layer.
 13. The base substrate according to claim 12, wherein the support substrate is a sapphire substrate.
 14. The base substrate according to claim 1, wherein the orientation layer is a heteroepitaxial growth layer of a sapphire substrate.
 15. A method for producing the base substrate according to claim 1, comprising: providing a sapphire substrate; forming an orientation precursor layer on a surface of the sapphire substrate, wherein the orientation precursor layer contains a material having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire, or a material capable of having a corundum-type crystal structure having an a-axis length and/or c-axis length larger than that of sapphire by heat treatment; and heat-treating the sapphire substrate and the orientation precursor layer at a temperature of 1000° C. or greater. 